U.S. Pat. No. 5,754,222 issued May, 1998 to Daly et al., which is incorporated herein by reference, discloses a method for visually calibrating a display by performing a visual offset estimation; determining a display gamma using a spatially modulated target; and determining an additive colorimetric mixing matrix using a neutral identification process. The step of determining the colorimetric mixing matrix relies on the visual identification of a neutral. In addition, their approach relies on the viewer to specify the chromaticities of the display or that these data will be supplied by some other source. Their process teaches an approach where the chromaticities of the display are entered by the user or selected from a list. For many applications this is neither practical nor possible.
Another approach to characterizing a display is shown in U.S. Pat. No. 6,023,264 issued Feb. 8, 2000 to Gentile et al. who employ a different stimulus presentation technique from that of Daly et al. (U.S. Pat. No. 5,754,222), but the step of determining a colorimetric mixing matrix also relies on the visual identification of neutral. The Gentile et al. approach (U.S. Pat. No. 6,023,264) does not provide an approach for visually determining the chromaticities of the display. Since these prior art processes rely on the user to either have knowledge of the chromaticities of the display (e.g., from prior calorimetric measurement); assume some set of chromaticities based on the display type entered by the user; or assume some default chromaticity if not available or known by user, these characterizations are fundamentally incomplete.
Another approach to characterizing a display is shown in U.S. Pat. No. 5,638,117 issued Jun. 10, 1997 to Engeldrum et al. where a display patch is adjusted to match a reference card of known colorimetry. This process is very difficult to perform because it requires the viewer to make both luminance and hue adjustment which may or may not be familiar to the viewer. Also, it requires the viewer to obtain and maintain the reference card as well as make decisions regarding the ambient illumination conditions for the environment in which the display is used. This process is too subjective and has the potential for large characterization errors.
Another approach to characterizing a display is given by W. B. Cowan in “An Inexpensive Scheme for Calibration of a Colour Monitor in Terms of CIE Standard Coordinates,” Computer Graphics, Vol. 17, No. 3, 1983. This article teaches a process for display calibration that uses a mixture of instrumentation and visually based techniques to calibrate then characterize a CRT display to a know colorimetric state. In general, the visually based characterization and calibration steps were described as methods for verifying the state of calibration of the display. Cowan's model for a CRT display consists of a nonlinearity and a colorimetric mixing matrix. He describes a simple instrumentation based approach that could be used to determine the nonlinearities of the display and a set of visual checks that could be used to verify that the display's channel nonlinearities remained constant over time. These visual checks utilized spatially or temporally dithered luminance targets that provide a known luminance reference that can be used to verify the calibration of the display. Cowan does not teach a process to completely characterize the display's channel nonlinearities using a visual process, rather he teaches how a visual stimulus could be used to verify the stability of the display's channel nonlinearities over time.
Cowan describes a process for determining the gun normalization factors for a display using a process of heterochromatic flicker photometry or heterochromatic brightness matching. The gun normalization factors Cowan referred to are equivalent to the ratios of the luminances of the display's color channels. In general, Cowan indicates this process is not accurate and should be used as a check of the stability of the luminance ratios over time. Cowan does not teach a process for designing visual tests that make the process of determining the display's channel luminance ratios accurate enough for characterization purposes. Additionally, the process outlined by Cowan does not provide a visually based method for determining the chromaticities of the display's channels. Cowan provides techniques for obtaining the chromaticities of the display ranging from performing spectrophotometric measurements of the display's chromaticities to contacting the manufacture of the display for this information. As such, there is a need for developing a process for visually identifying the chromaticities of a display.
There is a need therefore for an improved method of visually characterizing a display that provides separate mechanisms for determining the channel nonlinearities, channel luminance ratios, and channel chromaticities.